Thin films of Si-added-MoSi2 were deposited on Si3N4 or alumina substrate using an RF magnetron sputtering with a target made of MoSi2 and Si powders. The thin film consisted of a mixture of Si and MoSi2. The resistance of MoSi2-thin-film heaters fabricated using Si3N4 substrate heated for a long time in air increased with increasing heating-time at temperatures near 420°C due to generation of MoO3, which is an insulator, in thin film due to the oxidation of Mo in MoSi2. On the other hand, oxidation-resistant capability of thin-film heater fabricated by adding Si to MoSi2 was drastically improved compared with MoSi2 thin-film heater, because Si particles or the Si layer on a MoSi2 particle prevented the oxidation of MoSi2. The oxidation-resistant capability was excellent in air at high temperatures near 480°C for a long heating-time exceeding 950h.